I know another startup that was faced with a similar issue (they have since been acquired, proving that this does, indeed, work)--they decided to split it 49.9/

I know another startup that was faced with a similar issue (they have since been acquired, proving that this does, indeed, work)--they decided to split it 49.9/50.1 with the person who thought of the idea first getting the majority. Naturally if you have thousands of shares, just take one from the person who's going to get less equity and give it to yourself (assuming all shares have equal voting rights). Another alternative is to split the equity 50/50, but have the voting rights divided differently. This is a pain in the ass because you have to insert a bunch of (extra) legal mumbo jumbo in the operating agreement for tax purposes. A potential problem is if you both sort of thought of the idea together (i.e. it was an iterative process). Well, then, if you are adamant about a 50/50 split, you can always resort to third (neutral) party arbitration if a dispute should arise, but this is a pain. I would do it 49.9/50.1---one of you must have started this entire process.