Yes if you mean just going to view and have a chat with the owner, maybe a supervised 'trial' ride, then I think that's fair enough to be seeing/considering more than 1 at the same time, probably most people do.
If you mean starting a extended trial period of say a month of sharing/part-loaning before committing long term, I think it's more questionable. If the owner is going to be putting a lot of effort into showing you round yard, explaining/demo-ing how they want jobs doing, explaining the care, quirks, routines of the horse and all their stuff, introducing you to yard people, showing you hacking routes, supervising your first rides (all of which and more I do with my sharers, it's quite a lot of effort for me at the start albeit it pays off in the long term), I think I'd be quite miffed if the person turned around and said thanks but I've found another horse I prefer - of course that's the risk you run with sharers, they can cease the arrangement at any time for any reason, and I'd feel differently if their circumstances changed or they want to stop because they really aren't getting along with the horse. If you weren't upfront with me that you were 'trialling' several horses at once I might be pissed off and feel I couldn't trust you TBH, but on the other hand if you said that's what you were doing I probably wouldn't agree to the 'trial' in the first place so I can see your dilemma!
I think probably for anything more than single/initial meeting with the horses owner, you do need to be honest with them with where you are at, or you risk being called a timewaster or a joy rider (this does happen when you have a horse for sale or loan, people turn up sometimes multiple times with no intention of taking the horse, they just want a free ride/lesson for themselves/their kids. You even get CFs asking if they can ride the horse at a show (not comp horse, average kids pony) or take it on a month's trial so their kid can go to PC camp 
. Not saying you are remotely comparable to this but owners can be a bit on the defensive if they've had this kind of experience!