Circuit analysis of active LP-filter with 2 OP amps
I'd do it like this and sorry I can't make out the figures in your picture: -



You have to remember that the op-amps are just unity gain buffers and there is in fact only one unknown voltage and that is what I have called Va in the picture above.

All that's left is group together the terms for Vout and Vin to make your transfer function. Can you do this?