I have a portfolio that I manage for my 2 children that currently has only two stocks in it - FGX (Australian market exposure) and MGF (for Global market exposure) - that's Magellan Global Fund not Magellan Financial Group (which is the manager of MGF). My daughter is older than my son and had more saved up so she has contributed more into it than my son has, so I just keep the percentages written down (68.6% and 31.4% at this point), so I know how much of the portfolio's market value each of them owns at any given time. Any withdrawals by one and not the other would change those percentages, so I would make that adjustment at that time, so if my daughter wanted to sell $5000 worth of stock and take that out as cash (for example), I would start by working out the dollar value owned by each of them just prior to the sale going through, then recalculate the percentages based on the new dollar value each child owns after the sale proceeds have been deducted from my daughter's prior dollar balance, i.e. dividing my daughter's new dollar value by the total portfolio dollar value after the sale, then deducting that percentage from 100% to give my son's new percentage ownership. To cross check that, I can also divide my son's dollar value (which will not change since the sale proceeds did not go to him) by the new post-sale portfolio market (dollar) value - which should give the same result as deducting my daughter's new percentage ownership from 100% - because there are only 2 kids in my case). Then repeat that process each time a sale/withdrawl is made. Same thing applies to top-ups. Every time one of them puts in more money I recalculate the percentage ownership of the portfolio that each of them has, starting with the market value on the day of the top-up, then adding the top up to that child's dollar value, then dividing that dollar value into the total portfolio market value including the top-up to get the new percentage, and that percentage deducted from 100% is the percentage the other child now owns of the portfolio.

Hope that makes sense. Every time a child has money added, their percentage ownership increases, and the other owners' percentages decreases, and every time a withdrawal is made, the percentage decreases for the child making the withdrawal which increases the ownership percentages of the remaining children, and it is all just based on the market value of the portfolio (in dollars) before and after the sale.

If you keep account of the percentage ownerships in this way, you can be flexible with when and how much is withdrawn, and it can differ from child to child depending on their circumstances and wishes at the time.

So in your case this is money that you are putting in, not money that they are putting in, so it's up to you how you work the starting percentages out. The fairest way is to say it's evenly split between them up until the first withdrawal I guess.

However, if you want to try to make sure that they end up with the same amount each, then maybe just track the withdrawls and make sure they end up being even in the end (once there's nothing left). That's not how it works in real life though. Obviously the earlier you take money out, the less chance the money has to compound and grow. So making sure they both end up with the same dollars regardless of when the money is withdrawn doesn't reinforce that lesson, but it all depends on why you're doing this and what you want to achieve I guess.