Running retargeting campaigns with multiple providers has a number of serious drawbacks. If you run with multiple providers, each provider will be bidding for the same spots on the same websites, driving up media costs and decreasing the chances each has to serve ads to your users. You may also run into difficulties effectively implementing frequency caps, as each retargeting provider will be operating independently.

If you're new to retargeting and you want to test the waters with different providers, it's more effective to run tests in subsequent months using one provider at a time. You'll have a better sense of which campaign actually performed better without skewing your results.