It is possible to use multiple exoskeletons in modular armor, and the speed bonus will stack additively. There is no speed limit; it is limited only by the amount of room available in the armor and by available power.

Do exoskeletons work on spidertron?

For example, personal laser defenses and personal roboports work from the driver's equipment grid, but exoskeletons and belt immunity equipment do not. (Instead they need to be equipped in the spidertron's own equipment grid.)

Where is the equipment grid Factorio?

Access the equipment grid by right-clicking on the armor.