Ask HN: How to deal with a prospective third co-founder?

We are two founders about 1000 hours into our startup (currently starting private beta), and are currently looking for a third co-founder.

We try to find someone who has (1) technical or business skills and (2) someone who is motivated, good to talk with and who will pull the project with us (we aim at a public release in 4 to 6 months).

Of course it's not easy to find a reliable co-founder if you don't know the person very well, so we figured if we find a promising candidate we could do something like this:

(1) Have a one month 'trial' period which either party can quit at any point. After the first month have a reflexion meeting and, if necessary, extend to a second trial month. If the new person quits during the trial period for whatever reason, we'd compensate his/her work with money.

(2) If everything works out and we have a third founder in the team, we'd start splitting shares equally based on the amount of work everyone puts into the project over the next 3 to 5 years.

One idea how we could split shares based on the work: Have a web-based tool where once a week everyone enters the hours (and topics) spent during the last week. If three people put 1000 hours a year into the project over five years, it's roughly 15000 hours in total. This would suggest 1% for 150 hours.

How would you handle such a situation?

Any ideas, suggestions, experiences would be highly appreciated!