Creating an input form where 2 related values always need to be coupled together (e.g. weight value + weight units).

Creating an input form where 2 related values always need to be coupled together (e.g. weight value + weight units).

The problem I'm trying to solve is that I don't want the fields to look like discrete, separate fields from the user's end. 

On a website, when you enter related values like your height & height units, or weight & weight units, typically there's a dropdown next to the numeric field which lets you quickly calculate it.

What would be the best way to reproduce this on Zoho CRM's input fields? I could of course make 2 separate fields, but my concern is that it would look strange from the end-users POV. How could I show it in a way that it would be clear that those fields were related?

Also, how would validations play with this (e.g. when the weight units are kg, the minimum value is 30kg, but when the weight units are lbs, then the minimum is 66lbs).