But what they can't do by RAW, much to my extreme annoyance, is act AT THE SAME TIME.
By strict RAW, during combat two characters cannot move together (example: one is carrying a silence device to cover the sound made by both as they move; or both are under an Invisibility Cloak).
Lanefan
I ran into that issue last night. In the midst of a battle, I had a Nalfeshnee and a Glabrezu working together to pick up a stone altar that would have been difficult for either alone, and then they were going to carry it out the area of the silence spells that the PCs had been spamming to prevent them from completing a summoning ritual to open a gate to the Abyss.
I had one monster ready his action to help the other pick it up (though I suppose he wouldn't have had to ready, since RAW would allow him to help his ally with a check). Once they were carrying it between the two of them, I was going to let one creature ready his action to dash with the other character, and/or
maybe even let him use his movement with it.
Technically, they couldn't even do the simultaneous dashing, but I felt that being able to carry an item together was a common sense enough capability to make a ruling allowing it.
<End Point, Enter Story>
It never came up, because the Nalfeshnee's hit points got dangerously low and they had to change tactics slightly. The silenced Glabrezu mouthed his intention to the Nalfeshnee (both of them having Intelligence 19, I figured conveying simple information that way was justified) took his action to shift his part of the weight to the Nalfeshnee (I ruled that no check was called for, since the Nalfeshnee was probably strong enough to stand there holding it by himself), and used his move to start moving away. The Nalfeshnee then used his innate ability to teleport himself and any object he was carrying a few hundred feet away.
There also happened to be a wood elf ranger PC standing on the stone altar he was carrying, slaughting him with a short sword and a dagger--but his teleport ability only took
objects not other creature with him, leaving the PC behind.
With some intelligent decision making on the part of the demons, the Glabrezu did manage to make it to the altar in time for them the finish the ritual that required both of them and open the gate. It was a close battle and a good time was had by all. Since there won't be any follow up to those characters, I explained what would now be happening regarding the gate if the game had continued and it was up to the players' imagination what they would have done and how things would have played out. (Most likely they would have successfully retreated at some point, leaving the demonic horde to ravage the countryside, since the conditions for closing the gate before it had spewed out the marilith and balor and the inevitable TPK were probably impossible to meet given the action economy (too many scattered manes to kill in time.))
I would have ruled things pretty much exactly the same had the characters been trying to carry something, and it really felt pretty smooth. Given this is the first time I found myself needing to make those sorts of ruling in 5e, that is very promising for the ease and smoothness of spur of the moment rulings when I get some more 5e experience.