The only way to transfer the miles to either of their accounts without paying a lot of money would be to transfer American Express MR points or SPG points to one of their FF accounts, as both of those programs allow the transfer of points to various travel programs.
Since you are a DL platinum, it would greatly benefit them if you used your own miles to book their tickets, since platinum medallions get free award changes. That way if anything comes up, they find better routing, decide to add a stop over, change desination, etc you would be able to change the award free of charge. You can use your own miles to book a ticket for anyone else, and you don't need to transfer them to that person first.
If they are planning on paying you back for the miles you are redeeming (by accumulating their own miles), then they will need to keep the miles in their account and similarly book a trip for you with their miles to avoid the large transfer fee. If they don't have PM or DM status, then the awards will incur a $150 change fee since the miles are coming from their accounts. Also, they can't pool their miles, so you would have to book an award on each account individually. Or if one or both of them have AMEX MR points or SPG points, those could be transfered to your SkyMiles account.