I would be most concerned about the impact of shocks to the printers while on the road. Shocks can make things break. The nice thing about original packaging is it's designed to absorb most typical shipment shocks. Obviously you should make sure anything that moves (print head, covers, etc) are secure.
Personally, I would use the original packaging as a top choice. If not that, I would take the printers off their stands and wrap them in a ton of bubble wrap to help absorb shocks.
Ultimately this comes down to how big of a risk the loss one or both of these printers would be to you. If you are willing to accept the slight risk that damage might occur then by all means, tape them down, strap them down, and be on your way. If replacing these printers might mean signficant financial hardship for you, then get a bigger U-Haul and add more padding. The more you add the higher the cost and lower the risk. You have to pick the appropriate balance to your situation.