You shouldn't run two pumps in line with each other.

One should sufice, if it is rated high enough to run your filter. As for connecting UV's, if you have it on the exit side of the filter, it will help prevent detritus or stuff in the water column from getting inside and clogging the UV filter. Check the minimum and maximum gph recommended for the UV. If the exit flow (gph) from your filter is within range for your UV, then let gravity run it through your UV back to your pond. If it is not enough flow, you need a bigger pump (but not one that is too big for your filter).

OR

Just run one pump for your filter and the other pump for your UV seperately. Its very important to check the gph recommendation for the UV. Too much or too little and it ain't gonna do squat for you! ....since you won't have the correct amount of contact time.