hard to say without having a diagram of where the switches are in relation to the lights.
If you can get a 12/2 and a 12/3 between the two switches, it's easy. The 12/3 will be the travelers and the switch leg back to the first light, the 12/2 will be the switch leg and the neutral for the second light, which will get spliced to the existing cable in the switch box. You already have the wires to the lights, one cable at each box. First, check the wire and breaker size, it's probably 14 gauge wire on a 15 amp breaker, if so it can be 14/3 and 14/2 and will be much easier to work with, especially when it comes to getting down the walls and into the boxes.
If you run it under the eaves you can't use Romex because it's not supposed to be run in wet locations and outside is a wet location whether or not it's in conduit. You could fish Romex out of the switches and into a weatherproof box at both ends and use THHN/THWN in conduit between the boxes though.
No. It's combining two separately switched lights using two different circuits into two lights that are controlled together using two three way switches and one circuit. If he can get a two wire and a three wire cable between the switches it's easy peasy from there.
There is probably a door to the outside from the bedroom with a switch that controls one light, then there is another door to the outside from the dining room with another switch that controls a different light. He wants to be able to turn on both lights at the same time from either switch location.
That's a pretty common scenario. OP needs to open up the boxes and investigate. There's a small possibility it was originally wired that was and somebody changed it.
Or use UF. Not fun, but avoids the jboxes.
I knew a guy that was quite proud of using the ground wire as the switch leg for his porch light on his detached garage. We worked together at an electrical engineering firm…….
I currently have 2 patio doors w a light above them on same wall that are independent of each other. I would like to connect them to gain more light and flexibility.
Where are you getting 7 wires through a conduit from????
I have two patio lights; one on either side of a sliding patio door. One is controlled by a switch at the door directly on the other side of the wall, and the other is controlled from a switch in the kitchen (about 10' away). I'd like to rewire them so that they are both controlled together by the switch at the sliding door and by a new switch that I want to install upstairs. Kind of like this:
