X-Realm Relationship: Is this possible

Not tested but I will say "no problem".  

If you can do a Connected Sync table from QuickBooks or SalesForce  or even a lowly csv file in the cloud, I'm sure that you can set up a connected table to an app in another realm.  It will be as much as an hour delayed as the refresh is at most once an hour, but that is likely good enough.

I believe you can accomplish this with script and an <iframe> by setting 

document.domain = "quickbase.com";

in both the first quickbase domain and the second quickbase domain referenced in the <iframe>.

https://developer.mozilla.org/en-US/docs/Web/Security/Same-origin_policy#Changing_origin

I haven't tried it in years but I am pretty sure it will still work (although there might be some differences between browsers).