[The Fifth Horseman]

Do not mount an ISO with any virtual drives if you want to use it in DOSBox!
Been there, done that, don't want to ever again: It may work, but will cause worse skipping than from the real drive.

DOSBox has its' own virtual drive and you should use this one.
As to how:
imgmount D c:\images\game.iso -t iso

Support for multiple ISO images has been added in v0.70 It works like that:
imgmount D c:\images\gameiso1.iso c:\images\gameiso2.iso -t iso
(You can mount more than two images, of course - this is just an example)

Once mounted, CTRL+F4 cycles between the images.

So if you have ISO1, ISO2 and ISO3 mounted in sequence, CTRL+F4 it will go from ISO1 to ISO2, next from ISO2 to ISO3 and when you tell it to cycle forward from ISO3, it will go back to ISO1.