Festival for 5 days is a good example benefit of solar. Accepting you own both batteries and you dont own the solar yet. You could just in the festival if sat away from your tent in the sun just connect a small solar panel to your phone? That also may fail your criteria as the solar panel is about the same weight and larger than the 26800 you dont want to carry.
Yes you can charge one from the other. As @TechnicallyWell said you waste energy in charging batteries from batteries to just make it smaller in the pocket.
The only potential issue I see is the smaller battery will enter a trickle charge period in its last 10% til full where its intake current is so low the 26800 may shut off the port, so you may struggle to get the smaller batter much above 85%-90% charged. If that is the problem then keep pressing the 26800 power button to wake it up and do it a few times to shove the smaller battery closer to 100%.