The correlation between two assets (be it crypto or normal financial) isn't something constant.

Sometimes it exists, sometimes not.

Having said this. The last year, based on my observation, the correlation between BTC and ETH was pretty tight. When one rose, the other dropped.

For some short period of times, however, we saw both (BTC and ETH) could rise.

Might happen, that one day (when the fiat money are massively taken from crypto) we could see how both decline.