Transferring points directly between two Marriott accounts is fairly easy upto 50K points per year. If you want/need to transfer more, then you either do the same as what the above posters are doing (not too hard once you have all the accounts and linkages set up.)

One other way relies on the fact that Marriott will allow more than 50K to be transferred between accounts when the points are needed for an award. So you could make a large hotel award reservation for however many nights would eat up all your existing Marriott points plus the points in your spouses (or someone else's) account. Call MR tell them you have an existing reservation (make sure the reservation can be cancelled of course.) They will email you a form to fill out with your accounts info and the other person's account info. It will need to be signed by both of you and then faxed back to Marriott. Once you've done that, they will transfer however many points is needed for that award. After they have done that, you can cancel the award with all points going back to your account.

Personally, linking and transferring through SPG accounts is an easier process and doesn't include having to call Starwood or Marriott to do. So that is the way I have been doing it myself recently. And as a Marriott LT Plat, I'm not really concerned with my LT balance any longer.

One point as ehallison states point transfers either direction between SPG and Marriott doesn't have any affect on Lifetime points accruals right now.
Transferring directly between two Marriott accounts will decrease the LT points for the source account while increasing the LT points for the recipient account.