they make all kinds of adapters.even a 1/4inch "Y" adapter could work.your local music store or an electronics supply store should carry something to fit your needs.

I'm assuming it's you playing both instruments, one at a time.

Don't try to connect both guitars at once. Use something like a stomp box switch to select which guitar to feed into your DI.

yeah how bout an A-B box.

This question was posted on the live sound forum as well. It was suggested that an active A/B box be used. You can't mix (2) piezo-type pick-ups together ahead of the preamplification stages.