iPhone13Pro eSIM: Can I have another SIM for a 2nd number?

Yes, both can be active. Go to Apple for instructions:

https://support.apple.com/en-us/HT209044

In this mode when roaming off and cellular data off the att number will act like is connected to WiFi. It will connect using the other numbers data plan or WiFi. For voice and sms. 

The phone has to be unlocked for this to work.  If the phone is locked to AT&T, both the eSIM and the physical SIM slot will only work with AT&T SIMs.  Also, you’ll need an international plan for using your AT&T number while in Japan, otherwise you’ll get hit for expensive roaming charges.

Thank you for your quick responses. This is great news!

While I am in Japan, I need to disable AT&T Celluar and just leave it as WiFi call only (I only need to know/check if someone left me a voice message and need to receive SMS to iMessage only when I am on WiFi), or I will get a high roaming charge. The Apple doc site didn't explain if this is possible. Do you know the answer to this?

I've seen people mention doing this.  You also apparently have to disable roaming for that ATT line.

The only way I know of to disable cellular is airplane mode and if you do put the phone in airplane mode, that applies to both lines. I’m pretty sure there is no way to disable cellular on the ATT line only. What you want to do is disable cellular data on the ATT line. Since incoming texts are free, you’ll only get hit with roaming charges if you answer a call on the ATT line and you are not on WiFi calling.

Oh OK. So, is there a way to specify which SIM to be used for the data? Basically, I can't let AT&T cellular data to be used while I am in Japan. It must use the Japan SIM for data all the time.

Hi @madflute, let's look into how your usage of data can be affected while traveling abroad. 

Use these International travel tips when managing data use while traveling outside of the country.

Hopefully this information helps, and thanks for reaching out to the Community.

Jeff, AT&T Community Specialist 

Thank you for your response @ATTHelp but that document won't help. I need to use data with Japan SIM only and not using AT&T cellular data at all. I need to know if that is possible. If not, my only option seems to buy another phone to use with Japan SIM.

So, is there a way to specify which SIM to be used for the data?

Yes. Both sims will use data if you let them. By not allowing the ATT sim to use data, you restrict data use to the Japanese sim. You want to turn automatic data switching off and turn data roaming off for the ATT sim. Read the Apple document again, I’m pretty sure it covered how to accomplish what you want.