If you are co-locating your subs -- putting them both next to each other -- then it's best to calibrate them together. But you should also calibrate them separately first. What I mean by that is that each should play equally loud. The ensure this, hook one up individually, send it a test tone, and measure the output with an SPL meter. Use the sub's volume dial to get it about 6dB lower than you want the final calibrated level to be. E.g. if you want your final sub level to be 85dB, set it to 79dB. Do the same for the second sub, adjusting its output to match the first, again using the sub's own volume dial
Now that both subs are playing equally loud, connect them both to your receiver and pretend they're just one sub, using only the level adjustments provided by the receiver.
Of course, you may find that the subs sound better together if the EP500 plays a little louder than the PWS450, or the other way around. Aren't variables great?