Although it is technically possible to work with more than one CSS, that doesn't mean that adding more accounts brings greater benefit.
In fact, according to Google, having multiple CSSs place ads on your behalf won't mean your offers will be repeated. Having two CSSs place bids on behalf of the same merchant will not have a significant impact as the winning offer and the price paid by the winning CSS will be the same as if those bids had been placed by one and the same CSS.
Google also adds that they avoid displaying the same ad for the same merchant twice. According to them, "a merchant will never be second-priced against itself in the auction for any offer, and that rule holds irrespective of the number, or identity, of the CSSs used by the merchant. Therefore, if two CSSs place bids on behalf of the same merchant, the winning offer and the price paid by the winning CSS will be the same as if those bids had been placed by one and the same CSS".
In the end, a merchant can use more than one CSS to promote their products. However, if we're referring to promote the same product across multiple CSSs, Google has standard procedures to prevent duplication of ads for the same offer which makes it irrelevant to have multiple CSSs.