I presume you don't mean exactly at the same moment... Might be a bit tricky, that.
Only one man can father a single child.
However, if the woman was to have "fraternal" twins (or triplets or more), each could be fathered by a different man, if she slept with both during the fertile period.
Fraternal twins is where two (or more) eggs are released at ovulation instead of the usual one. If one is fertilised by a sperm from one man, and the other by a sperm from another, the children would be half-siblings. Such twins are of course no more alike than normal siblings of different ages.
This could not happen with identical (monozygotic) twins. These come from one egg, fertilised by one sperm, which starts growing as one embryo. After a few cell divisions, for some reason the embryo breaks into two (or more) and each grows as a whole child.
Of course it's all much easier in those animals which normally have multiple births. Most of these ovulate many eggs at a time, and so the chances of different parentage in the litter are much higher.
A neighbour of ours had a young cat which had two kittens. One looked exactly like our black-and-white short-haired cat (who had not quite yet been "done"). The other kitten was a fluffy Persian cross, which surely could not have been his.
A few animals (such as armadillos) routinely have monozygotic babies, having identical octuplets etc. Don't know why.