Q: Can I use the 64-bit add-on together with the 32 bit Server component?
A: Yes, you can (in fact you must as the server component is mandatory). So on the machine that runs the Add-on you install the 64 bit B1 Client. On the server that runs the server-component you need to install the 32 bit DI-API (or the 32 bit B1 Client).
Q: Why do you only offer a 32-bit Server-component (and not a 64 bit as well)?
A: There are two reasons for this. If we were to offer both a 32-bit and 64-bit Server-component there is a chance the someone by mistake would install both of them on the same system and that would result in all server-work being done twice and on top of each other (aka a big mess). Since we can therefore only offer one bit-version and our statistics show that 75%+ are still using 32 bit versions we have decided that server-component is 32-bit only
You can read more about 32-bit vs 64-bit here: https://support.boyum-it.com/hc/en-us/articles/204926567-FAQ-32-vs-64-bit-Add-ons