Running two CSS partners at once

You are allowed more than one CSS. You are not allowed the same product through two of them at once. That single constraint decides the whole shape of a valid comparison.

The rule

Multiple certified CSS partners can place Shopping ads on your behalf at the same time. Any individual product, however, can only be submitted by one CSS at a time. A product offered through two will be served through one of them, not both, and not predictably.

The valid design: split the catalogue

1. Divide the catalogue into two halves that are genuinely comparable: similar categories, similar price bands, similar margins, similar historical volume. Alternating by product ID is a poor split; stratifying by category is a good one.
2. Put each half in its own Merchant Center account, and associate each account with a different CSS.
3. Mirror the campaign structure and bidding strategy across both halves.
4. Let it run long enough to clear the noise. Two weeks is usually the minimum, and a full purchase cycle is better.
5. Compare average CPC and impression share on comparable products, not total spend or total revenue.

What you will and will not find

You will find whether the association is actually live on both halves, and whether one partner’s implementation has a problem. You will not find a meaningful difference in the discount itself, because the auction terms are identical by regulatory requirement. If a test shows a large gap in CPC between two certified partners, the likeliest explanation is that the split was not comparable.

When it is worth the effort

Rarely, for a single merchant. The cost of maintaining two Merchant Centers usually exceeds what the test can tell you. It is worth it for agencies evaluating a partner across a client base, and for large catalogues where operational differences between partners, rather than auction differences, are what you are actually measuring.