What would the offspring be if 2 roan cows were crossed?

There wouldn't be any offspring. A cow cannot mate with a cow to get a calf. You must mate a cow with a BULL to get a calf.

So, if the question were reWorded to: "What would the offspring be if a roan cow was crossed with a roan bull?" or vice versa, then we could answer it this way:

Roan colouration in Shorthorns is a codominant trait, which means that one allele is not dominant nor recessive over another. So, the resulting offspring of a roan cross would be a mix of red calves, white calves, and roan calves.

The Punnet Square reveals the probability of one of three coat colours occurring in the subsequent cross:

Let R = red and W = white and RW = roan.

RW bull x RW cow gives us:

1 RR

2 RW

1 WW

In other Words, there's a 25% chance that the resulting calf would be red, a 50% chance that the resulting calf would be roan, and a 25% chance that the calf could be white.