First, an advanced panel alone won't suffice. While IC2's induction furnace uses only 1 EU/t to keep itself heated, the other upgraded machines come from a separate mod and behave differently. They need at least 3 EU/t each, so you're looking at 10 EU/t total minimum.
As for where the energy comes from, it would probably be split. If you want to make absolutely sure that the thermals only work if the solars can't keep up, you'll have to rig a little control mechanism.
- Hook the solars and thermal generators up to a batbox
- Hook the batbox up to the advanced machines
- Configure the batbox to "output redstone signal when empty"
- Lead a redstone wire to a splitter cable in the connection between the thermals and the batbox
- Make sure the splitter cable is open (not transmitting) while the batbox is not empty
As a result, when the batbox runs empty it will emit a signal and close the splitter cable, connecting the thermal generators and letting them supply power. In all other cases, only the solars will do the work. You will also have the benefit of the 40,000 EU in the batbox as a short-term buffer, in addition to the buffer of the advanced solars. With that setup and two advanced solars, you'll probably never see the termal generators needed at all except for very large batches of induction furnace uses early in the morning.