Switch – Combine Two Lights from Separate Breakers Into One Switch

This explanation assumes you are trying to control both sets of lights at the same time and not independently.

If they are on the same circuit, having separate hot wires to each switch is really redundant. The return wires from the switches are not.

There is no problem ganging both lighting fixtures on the same switch so long as the switch is rated for the power the fixtures draw. Most modern dimmers are rated 150, 300, 600 or 1000 watts. You need to make sure that the two fixtures, added together, are somewhat lower than the switch rating. If not, your gonna need a bigger switch!

All you need to do is hook one of the hot lines to the switch and cap the other hot line with a wirenut. Then pigtail both of the two switched lines (that return to the fixtures) to the switch output side. Do be sure to attach ground, for safety and to reduce the chance of random hum.

All of this assumes that the dimmers are suitable to the type of lighting (special dimmers are needed for many CFL and LED bulbs), that they are single pole (not three way), and there is no special feature requiring a neutral.

SUPPLEMENT

As Tester101 needs to regularly remind me, current code calls for a neutral wire at all switches. You have an extra wire in the switch box (the redundant hot) that could be converted to a neutral if you ever need it. You would need to switch it over at the fixture to the neutral white line, mark it at the fixture to show it is neutral (e.g., with white tape) and mark it at the switch box as neutral (also white tape).

Since you have two circuits (i.e. two home runs from the electrical panel) in play here, you can't "combine" the switches in the manner you're thinking of. This leaves you with two options:

1. You can buy a second RPLS540A EconoSwitch and install it in place of one of the 3-way switches for the outdoor lights as per the instructions. This is the simplest approach, but leaves you with two EconoSwitches to program and the possibility that your two sets of lights will wind up "out of sync".

2. If you have a neutral in the box for the single pole EconoSwitch, this also raises the possibility of a setup using a SPDT relay in place of one of the three-way switches with its coil controlled by the existing EconoSwitch; this would require running an extra cable from the box with the single pole switch to the box with the three-way switch for the outdoor lights, though. You would also need to swap the existing EconoSwitch for a RPLS740B: the RPLS540A you have passes the power it uses to operate through the load, while the RPLS740B uses a neutral wire of its own to get its operating electricity "back home", so to speak.