Can you make tres leches cake with two leches
You can drench the cake with any liquid you like! :) Just don't call it 'three milks'. In some cookbooks this cake next to recipes for English trifle (sherry, jam, pastry cream, fruit), and Italian tiramisu (coffee, etc). Anyways, the sweetened condensed millk is the part with a distinctive character. I'd use that, with as much light cream as I had, and stretch it with the milk if needed.
Do you happen to have some eggnog left over from the holidays?
The proportions given in the 1997 Joy of Cooking version is 1 can sweetened condensed, 1 can evaporated, 1c heavy cream (for 9x9 cake). I considered making that for Christmas, but instead made a trifle.
And also a Puerto Rican nog from 'Daisy Cooks', which used 1 can sweetened condensed, 1 evaporated, 1 can cream of coconut (the sweet pina colada stuff), some eggs (or egg beaters). Without the 1 1/2 c of rum this mix was rather heavy and sweet. I ended up using the leftovers to sweeten my Vietnamese style coffee.
The Joy of Cooking version is also available from Saveur
It is clearly something that can lend itself to all kinds of variations. As with key lime pie, the use of the canned milks suggest roots in a tropical climate where fresh milk and cream were hard to get.