The one thing you want to avoid is having both throttles connected electrically to the same track at the same time. That situation can cause a back feed from one to the other throttle and damage them. You can wire one throttle to the mainline, and one throttle to the yard. Then on the passing siding that serves as your yard lead or entrance to the yard wire in a double pole/double throw switch then if your are coming in from the mainline once the locomotives are entirely on the siding, flip the switch to yard power, and run the train on in with the yard throttle. Do the opposite to leave the yard with a train. You will also need one or more tracks in the yard where you can park a locomotive and bring in a train without having the parked locomotive move. You can do this with power routing turnouts, but I've never wired a power routing turnout, so I can't tell you how. The other way to do it is to put an insulated joiner on one rail just past the turnout on the track you want to isolate. You then wire in a simple single pole/single throw (on-off) switch with the power coming from the same rail on the "hot" side of the turnout. The power out of the switch goes to the other side of the insulated rail joiner. When the switch is turned on, that track section is powered so you can move that locomotive. When the switch is turned off, that track section is dead so a locomotive will stay parked. I'm not sure what the spec is on your MRC Tech 4 power packs. I have MRC Tech 2 2400's. My throttles will operate 3 Athearn locomotives. They will probably run 5-6 can powered locos like Kato or Atlas. Lifelike P2K probably fall in between because of the power demands of their lighting packages. If you have a dozen or more locomotives parked in an engine service facility and all are on live tracks when you turn on your power pack, you may get an overload condition. You want to have sections of track a little over one locomotive in length set up with on-off switches so that you can park the locos and still operate the railroad.