Yes this is allowed, and I have had several realtors willing to play ball with me in these situations.
The key is to have an option contract that includes language to let you do that. Mine says something to the effect of SELLER hereby grants buyer immediate access to property and permission to market property, including listing on the MLS and entering into contracts for purchase and sale.
it's important to go over this one with your seller in advance, otherwise they're gonna be pissed when they see their house listed for $50K more than your option price.
Just look them squarely in the eye and let them know this is how you do business, and it's what it takes for you to help them. If that's an essential part of your exit strategy, don't for a moment slack off and let them get away with compromising the rules you've set for yourself. You will regret this later.
Ask me how i know this!!! (actually don't)
Then again, if they're motivated enough, they'll do it.
Paul