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On topic, strap the bike the way the picture above has shown and you'll have no problems. If you wanna go completely nuts you can build a 2x4 frame to mount a wheel chock to and strap the bike down to that. Then run two straps out to the rub rails on the side (make sure they pull the bike forward). I did this once when hauling 6 bikes to CT from MT in a single uhaul and had no issues but for 1 it's massive overkill.
As for the 5x8 with two bikes, no issues at all. My old HF 4x8 routinely carried 2 bikes side by side, both pointing forward. Chocks were mounted fairly wide apart to allow the bikes ample clearance on the bars. I bolted angle iron to the front of the trailer to extend the width a foot and a half or so on both sides and put eyebolts in the angle iron. That allowed a good angle on the outside strap of both bikes to secure them properly. Worked great for the roughly 10-12k miles I put on that trailer.