Can Restylane Be Combined with Other Fillers to Lessen Side Effects?

Restylane (and other hyaluronic acid fillers) work in 2 ways:

They fill the area with the actual volume that is injected.
They absorb fluid to create even more fullness.

If you have severe swelling after Restylane injections, it is remotely possible that you have a true allergy to the Restylane.
More likely is that the Restylane is absorbing fluid and giving you more fullness in addition to the actual volume that was injected. You will be more susceptible to this under the eyes where the thin eyelid skin does not conceal swelling.
When I inject Restylane, especially under the eyes, I under-correct due to this tendency to absorb fluid. Then, after 2 weeks, if you still need more, we can always add more.
Juvederm tends to cause more swelling than Restylane, possibly because of the stronger cross-linking.

Swelling is a common occurrence after any injection. The amount of swelling is variable but is generally minimal and lasts only a couple of days. Multiple different fillers can be combined for certain purposes. The effect though, would be the same if you use one filler or multiple fillers.
If you do experience 'severe' swelling after a Restylane injection, this may indicate an allergic reaction to the Restylane and likely to all hyaluronic acid compounds. It would be very important to have this checked by the administering physician to be sure that you are treated appropriately. If this were to happen, it would likely happen soon after the injection, likely while you were still in the office.