TL;DR - I've done it. One had a successful exit, the other bombed. Spend all your time, energy and passion into one and make it win. I used to run two startups

TL;DR - I've done it. One had a successful exit, the other bombed. Spend all your time, energy and passion into one and make it win. I used to run two startups in parallel - 1) an online b2c startup - review and recommendation site for local businesses and 2) manufacturing and installation of bird spikes. Not only were both businesses completely different in all aspects, they were also run out of two different cities. I used to run the online businesses, while the manufacturing one I handed over to a friend and would touch base with him very week to check progress and help troubleshoot sales, marketing etc. The online startup was a roaring success. We exited successfully in just 3 years of starting up. The manufacturing one failed - the friend lost interest, I couldn't manage the day to day affairs and it ended up being a disaster. So don't do two things in parallel.