I have received two offers, both verbal awaiting specific details about pay etc. I have a clear preference for one of them but they have been quite slow in the process, both jobs are through headhunters. How do I inform the preferred headhunter of my other offer so they can tell the company speed up the process, I don't want to accept the other offer and then back out later, but I'm worried I may have to end up doing that. Any other suggestions how to handle the situation? The other headhunter is saying that if I accept their verbal offer, I need to withdraw all my other offers/interviews, but what happens if it falls through. How can I delay this or should I, awaiting for my preferred offer?
Any other suggestions how to handle the situation?
I would immediately notify both headhunters that you have other offers, and would like their offers in writing as quickly as possible so you can make your final decision.
Prepare yourself to make a quick decision, once you have formal offers. Think through the both situations ahead of time, get whatever other information you need to make your decision (benefits information, etc as suggested by @David), as you may not have a chance to hold off one position in hopes the other comes through.
I would never advise someone to accept one position with the intent of quickly backing out if the preferred position comes through. I think that approach lacks professionalism, and can ruin a reputation.
You are in an enviable position.
Since they haven't moved quickly, you are under no obligation to move quickly yourself. As such, you have time to "ponder" both offers, at least as far as they are concerned.
I would tell them both that you are waiting on another offer before making a decision. That typically motivates people to move faster because most people will take the first job that comes along of the two.
If the offer comes for the job you don't prefer first, tell the one you do get the offer from that you'll make the decision in 24 or 48 hours and then call the one you do want more and give them a chance to counter it and with a time limit.