Looks like Android 12 will let you use two SIMs on Pixel 4a 5G and Pixel 5 without any trade off
Unlike the Google Pixel 3a and later phones, the Pixel 4a 5G and Pixel 5 don't let you take full advantage of the Dual SIM Dual Standby (DSDS) option. This could change with Android 12, reports 9to5Google, citing a Reddit post.
DSDS lets you use two SIM cards simultaneously - a physical SIM card and an eSIM. Users can decide which SIM to use for which action, which means you can use one carrier for calls and texts, and another for data.
On the Pixel 4a 5G and Pixel 5, if you use two SIMs at the same time, they fall back to 4G networks. To connect to 5G on these phones, you need to turn off the SIM that isn't set to use data temporarily. Your options including removing a card, deleting a profile, or disabling a SIM through the Settings app.
It was earlier believed that Google would enable Dual SIM Dual Standby 5G support via an Android 11 update. A code change posted back in November 2020 to the Android Open Source Project (AOSP) had suggested that Google's new phones would be able to connect to 5G networks in DSDS by December. The February security patch is out now and it does not offer DSDS 5G support.
That's apparently because the functionality is not due to arrive until the next major OS update.
A user on Android 12 Developer Preview 1 has shown a Pixel 5 with DSDS and 5G. The phone allegedly has DSDS active and '5G' can be seen in the status bar.