Both female, or is one male? People have had unintended breedings result from putting a male and female together for only a few minutes. Assuming that's not an issue, and that they're fairly similar in size, I wouldn't expect major issues. Obviously don't do it if one still has the smell of food on it, etc.
That said, why not put one in the "holding tank," clean its enclosure, put it back, and then repeat the process with the other snake? It's not like one person can simultaneously clean two enclosures anyway, right?