So you are going to have to do some math I am afraid, because you wanted enough power and light enough. So two Powercore II 6700, or two Powercore 10000, or even more. Conflicts between lightness and enough power.
Firstly you need to work out your power needs. Do you know how to do that?
- take each item, for example let us using the iPhone 6S, it has a 1715 mAH battery. How many days will that last you from full to empty? If it lasts 1 day hence that needs 1715mAh/day. If it lasted 2 days then its 1715/2 = 858mAh/day.
- do that same for all your items.
- add it up, you'll get a number. For the explanation of the math let's say this added upto say 2000mAh/day of demand. You will get a different number, subsitute the 2000mAh/day with what you got from the above.
- Then use a rough method of 3/2 of that to allow for the inefficiencies and the different voltages involve, some cable loss to get the input power. If you doubt this 3/2 then look at this video which explains it long-hand.
- Then your 2000 becomes 3/2x2000=3000mAh so you need 3000mAh/day of actual power.
- Then use a nominal 3.7V to convert A to Watts. This has been factored in the 3/2 above so 3000mAh/day becomes 11100mWh/day. Divide by 1000 to convert mW to W so this is 11.1Wh/day
- What number you get this stage is what is your "daily need" I use below.
Then let's discuss solar and the panel you link to. That panel is actually a good panel, it is efficient and unfolded quite large, but like many solar panels it tends to, cough, exaggerate, what you actually get. As rough guide it is not a 21W panel it is a 10W-11W panel, I will use 10W in the calculations below. Below I show what you can get from solar.
- in good sunshine you get 10W, and then multiply by the time you get. So if you propped it up from sunrise as you cooked breakfast and packed your equipment, and then at any lunch or rest stops, and then as you put unpacked and got your tent upright, say you got 4 or 5 hours of good sunshine per day. Then you get 40Wh to 50Wh on such a day. But some days are cloudy or rainy so your average over your 6 weeks would be lower at times.
- So let's say on average over 6 weeks you got 30Wh/day. Some days more, some days less.
Then let's combine your needs with the solar.
- In the example above I got 11Wh/day needs, and above I got 30Wh/day of solar. So as 30W is a lot more than 11W that means on average you will easily keep all devices going because the few hours/day is easily able to keep up. Hence in this example you need to carry just enough Powercore to store up the sun during the day and carry you through the days when no solar (cloudy, etc).
- So let's say your daily needs now are that 11Wh/day and you wanted to survive 2 days without solar. Then you needed to carry 22Wh of Powercore. You said you wanted redundancy so assume 2 similar products, so 22/2 = 11Wh, so you are seeking a 11Wh battery
- so then divide by the 3.7V and then you get to 3Ah Powercore, or a 3000mAh Powercore which is the Mini+ model.
If you wanted a gut instinct non-math advice then I would suggest two Powercore II 6700, but recommend doing the math it might be need two Powercore 10000 for a bit more weight.
The 21W solar panel is 10W-21W so that is 2A at USB 5V to a bit more. In really good conditions, perfect blue sky, high altitude (stronger UV), and perfect alignment you might find the Watts bursts up to more than 2A. I have seen some get 14W (2.8A). So if you wanted to allow to not waste those perfect conditions I do not recommend the example of two Powercore+ Mini as they max at 1A input each so two of them is 2A input max so you may be wasting some solar energy. The 2A input products are the Powercore Slim 5000, the Powercore 5000, the Powercore II 6700 and then all of the bigger ones (10000, upwards) so to not waste solar energy and keep weight down then these your sweetspot minimums from lightest to heaviest:
- Powercore II 6700 + Powercore+ Mini. 3A input, 10000mAh total capacity
- Powercore II 6700 quantity two, 4A input 13000mAh total capacity
- Powercore II 6700 + Powercore 10000, 4A input, 16000mAh total
- Powercore 10000 quantity two, 4A input, 20000mAh total.
I cannot imagine you would need more than that.
Now you did say you wanted a two-port Powerbank, well these are all larger, and yet you said you wanted to be as light as possible. From the calculations you will probably conclude you are carrying too much power and too much weight to get two dual-socket Powerbanks. The smallest dual-socket is the Powercore 10040 but observe it is 240g vs the Powercore 1000 180g, and the Powercore II 6700 140g.
If you do find the Powercore you decide to buy easily keep up, are nearly always full, you could save a bit of time by attaching your devices directly, rather than attaching Powercore to solar, recharge Powercore, attach Powercore to device. If you were counting every ounce then you trade weight for a little bit more faff so go for smaller Powercore and then when they are fully charged then attach your devices to recharge them.
The one idea you did not ask is to have NO Powerbank and recharge all your devices off solar. The problem with this method is that some of your items recharge slowly, they have smaller internal batteries so if you plug in directly to the panel you would need to leave them attached for hours. It will save you weight though. So at least one Powerbank which ingests energy faster is recommended to then attach that Powerbank to your slowly recharging devices.