putting two side by side on a trailer is probably not going to work since you will probably need at least 8 feet
If you are taking two a 20 foot trailer would be the safe way to go, youd have room for two
you might be able to get away with a smaller one, if you put both sleds on diagonal you should be able to use an 18 footer or a 16 footer.
If you plan on using a landsape trailer you need to have the right size vehicle to tow it, anything less than a half ton pick up is not going to handle it.