i was in the exact same situation in a few cases, even not too long ago, just on the other side. i was the ceo.
we are in webapp development, so basically everything related to that belongs to the company.
now, one of the guys did a really cool tool in his freetime and showed it to us a couple of weeks later.
my first question to him was: how much do you want for it? even though this really had no connection to any of the things we did at the time, not even the plans we had for the future.
still, we agreed on the price and he got paid out immediately.
exactly one month later a client came with somewhat similar needs for his website, so we sold it without any update.
since the the sales price was quite above the aquisition cost, and we had to do no work on it, the guy got a compensation later to become 50/50 with the company in that subject.
me buying stuff from co-workers happened many times, i think it's great when a company has an offer or an opportunity for you, other than your regular job.
it might not change a thing during workdays, though. :)
anyway, i suggest you go for it, try to get best offer you can get, but if you name the price make sure you don't look like a fool by settling in a fraction of your first offer, so price it right!
the technical cofounder has to have knowledge about server side code, database, etc, so at least one of you is totally in picture of what's going on.
also can cooperate with new employees, subcontractors on a technical level, and certainly can act instantly in a given situation...and probably won't complain about working round the clock :)
with such team, i would feel much safer as an investor too.
