Do I understand you correctly that you have a dimmer upstairs and a dimmer down stairs?
If so, then your lights will never be any brighter than the dimmer that is set to the lower setting.
The easiest way to determine what wire goes where, is to get a meter. Set it on ac volts, over 100 volts.. Attach one end to ground, look for voltage on the other wires. When you find power, mark that wire. Then go flip the other switch. Back at the meter, look for power again.
IF you find power on a different wire, that is the other traveler. The remaining wire is the common.
IF the power is still on the same wire, then that is the common wire.
2 dimmers on one 3-way circuit is just not a good idea. You need to decide which end is most convenient to adjust the lighting from,
Which end you will just want to turn the lights on from.. in a 'set and forget' dimmer situation, make that dimmer the least convenient to operate.
Either way, If you don't have a meter, a test light will also work.
Welcome to the forum!