I cut wood planks just long enough to hold up both my tandem axle trailer tires at the same time. On each end, I cut a 45-degree angle so the boards don’t flip up when the tire hits it. I can use one board or two depending on the rise needed.
To keep the top board from slipping, I drilled three holes through both boards and installed some three-inch lag bolts. The longboards don’t sink as easily in soft soils. If I need even more I have a pair of 2" solid rubber pads I add.
As far as having only one tire up on a board I've done it often and no harm, even done it for months at a time at our summer RV spot and no harm done as far as I can tell. Your mileage may vary though. my rig uses leaf springs and a center equalizer. Given that, I doubt having one tire up 1 1/2 inch from the other would do anything, but it helps us if the level is just in between a full board.