Just noticed this one a bit late. As others said, as long as the conventional engine isn't pushing or pulling the LC+ engine you shouldn't have any problems. Even a small amount of push or pull can cause problems over time as it will put added stress on the drive train as well as the electronics. With the LC+ speed control it will likely vary between pushing and pulling the traditional engine as it speeds and slows with varying track power. I wouldn't be too worried about it to run once in a while, but I don't think it's a good idea to do for too long.
As for lower voltage harming the LC+ engine, this could actually be an issue if running the LC+ engine at higher speeds, or pulling heavy loads, on lower AC voltages as the engines power supply has to 'work harder'. The lower the AC voltage, the more work the power supply has to do to 'fill up the tank' so to speak. I wouldn't be too concerned about this for normal use, but it might cause a failure sooner than running at 18VAC would. Big L chose to use some cheap capacitors in many LC/+ engines, and if longevity is the goal, I wouldn't make them work harder than they need to. From the testing I've done 13VAC seems to be the lowest voltage that has no effect on the engine's functionality. (incidentally, at 13VAC, the engine has the same amount of 'power' as it does off of 18VDC)