I've done this twice with different boats and diff trailers. It calls
for some ingenuity but it isnt too hard. It is easy if the jacks can
go outboard of the trailer. Tie a rope to the toe rail (use a
carabiner) and tie the other end to a come-along and to a tree. Pull
the boat over a little sideways so you can get a jackstand under the
other side. Repeat on otehr side.
If the jacks must go inboard (within the footprint of the trailer),
then use stacks of cinderblocks and pieces of 2X6 stacked. You muist
stack them so you get weith off the trailer, then pull forward till the
axle almost hits a stack, restack on the other side of the axle.
Finally, replace your blocks with the jackstands.
Some boat movers have special trailers WITHOUT AXLES THAT GO ALL THE
WAY ACROSS just for this reason. Instead, the trailer has removable
sections in front and rear.