Aligning the BOOMS of two antennas at 90 degrees to each other
does NOT guarantee a lack of interaction.
For example, a 24 ft boom end loaded by 20M elements such as
the Telrex 3L20 or Hy-gain 204BA is self resonant around 7.5 MHz.
I expect the boom of your LPA will be self resonant somewhat lower.
It is usually best to just align them in the same direction with as much
spacing as possible and take the consequences.
A 100 ft tower is perfect for a single 40M antenna but a little less than
optimum on 80M. Very high 40M antennas (140 to 200 ft) are great for
LP and skew path but are usually beaten out by lower (70 to 120 ft)
high 40M antennas in the late afternoon/evening openings.
N6BT has one of these (2X2 80/40) antennas at his home QTH.
de Tom - N4KG