Can I move 240' and attack in one round by using two riding horses?

Question

The mounted combat rules aren't very clear and I haven't had much luck finding the answers online. For the sake of this question, let's assume the mount is a riding horse. Can it be used as follows:

Can a mounted rider ride a horse its full distance of 120' (it's movement+dash action), then dismount (for half her move), then mount another horse (with the other half of her move), then ride the second horse its full distance (another 120'), and then attack with her action?

Answer 1

Short simple answer here. No you can't, you can only mount or dismount once per turn. Player's Handbook, page 198:

Mounting and Dismounting

Once during your move, you can mount a creature that is within 5 feet of you or dismount.

Answer 2

This question is an example of how a DM should deal the concept of Rules Lawyer vs Playability in the game.

Given the way the rules are written, there's a loophole that can be exploited. Yes, it's possible to string those actions together.

However, logically, take a minute to think about what you ask. In a round (which is 6 seconds), can you: move 120', change mounts, then move another 120', after which the rider then makes some attack?

The fastest horse in the world can do 2 furlongs (1320 ft) in 20.6 seconds, which translates into 384' movement rate. That's at full gallop, not from a standing start. Even presuming the character is going to leap between saddles of moving horses as the dismount/mount action, adding an attack at the end of the round stretches believability beyond the breaking point.

Good DM-ing in D&D is not just about adhering solely to the letter of the law; this will absolutely fail, because there's no possible way to write rules that cover all possible situations, and questions like yours frequently pop up when defined actions result in unintended outcomes. Logic and consistency is the path to take, and being free to ignore the strict letter of the rules is a key part of being a good DM. Not capriciously, but when the rules clearly aren't rational. This is such a case.

TL;DR: Yes, it's possible, according to the letter of the law. NO, a DM should not allow this kind of rules exploit. Under no circumstance should additional movement be allowed after the character takes their full movement themselves, unless they decided to forgo any other actions in the round.

Answer 3

I'd say that this is against the spirit of the rules and an exploit, and disallow it. Don't forget that the DM has much more opportunity to exploit the rules than you do, and if you're playing strictly to the letter then you're gonna run into problems with any system.

It highly reminds of of the peasant rail gun, which to quickly summarize:

1. Get roughly 2200 peasants to stand in a line.
2. Peasant #1 has a 5 foot pole. #2200 readies an action to throw it. 2-2199 ready an action to pass it to the peasant in front of them.
3. Peasant #1 passes it on, each peasant activates their readied action.
4. The pole in one round moves at roughly 550 m/s, breaking the sound barrier.

While strictly permitted by the rules, this is clearly an impossible situation. If you want the kind of game where this is allowed, it can get very silly very quickly.