What is the signal the difference?
Follow this guide and record the numbers you see...
If the difference is within ~10db there's not much to worry about.
Now if you see -130 and -80 that is an issue. (talk to your carrier)
But if you see -60 and -75, both phones are operating normally.
What are the iPhones in question?
Are you both on the same network? (what network(s) are you on?)
What data connection is available on each phone? (3G/4G/LTE)
Are the iOS and Carrier settings up-to-date?
Lets see if the OP follows through with the report...
It may be a case of a tower that is mid-upgrade, or a phone that's not compatible with the latest and greatest technology.
The signals in the house may be 3G 850 and HSPA+/LTE 1900/2100mhz.
Those different bands will be coming in with different strengths, and that's what I'm suspecting.