One solution is split stock ownership in the manner you would without any funding. Then determine the amount of seed capital you will raise and the cost of equity or implied valuation. Then attempt to actually raise that money from an outside investor. If the implied valuation needs adjustment before there are any takers, then use that figure for any stock sold in your seed round. Whether you actually take outside money or not (or sell stock in part to an outside party), let that valuation govern the stock sale. And whether co-founder(s) partake(s) in that round, its treated a separate matter and a separate dilutive transaction from your "founder's stock."

Of course often when the amounts involved are relatively* small (*depends on the venture and the people involved), sometimes the costs that one co-founder incurs is handled in an informal way, and is part of the relatively even contribution between co-founders. Partnerships are never equal (contributory aspect) all of the time anyway. Sometimes you pull more weight, sometimes it's the other(s), maybe it never evens out -- but if it's close enough it's a better working dynamic to leave it simple.