It can be shown that the Eligius pool is likely multiple block chains by looking at the coinbase of the generated bitcoin transactions. Here's a coinbase from an Eligius  bitcoin block that was used for namecoin only merge mining:


The details of the coinbase are available in the merged mining specification. From this you can see that the number of entries in the aux merkle tree is 1. This means up to 1 other chain is being mined. That's probably namecoin. You can look up the hash of the namecoin block. It's hash 4976cbf9003c0705e29e581038d3ddbe260a5d324c2b17b24779b6ca48c4f832 or  block 36067.

Now look at a slightly later block. Say 160499:


The aux merkle tree size is 4. So up to four other alt chains are being mined. Why the increase if it's only namecoin being mined? Also note that the hash in the coinbase no longer corresponds to a namecoin block. This is another sign multiple chains are being mined. This is during the period that i0coin and ixcoin hash rates increased.

Now a more recent block:

The aux merkle tree size here is 8. Why the increase? Coiled Coin and Devcoin are both active at this time and would have required the aux merkle size to be upped. I got the Eligius found blocks from here.