You can't use a splitter to combine two different phone lines. Ever~! She can't even use a two line phone without some rube goldberg kludged wiring.What she'll end up having to do is either buy a second handset for the MJ or unplug the one she has and plug into the MJ to make outgoing calls.
Oh how foolish of me! Of course you cannot!I'd delved down into MJ too far to even realize what the question implied. Thanks for not ranting.