Cool code tags used, thanks.
Between "LCD 2 lines of text updating at the same time" and "However I can't seem to get it to work because the text keeps on glitching out on one line." the issue remains vague.
Are you using an Uno?
double is irrelevant on an Uno and is the same as float.
I found that with setting floats to a default or hard coded value to use the 'f' to designate a float. Like this degree_inside = 0.0f; or you may find your floats are converted to ints.
does that pad a bunch of spaces into the LCD, then you add info to those padded spaces, no?
Your code waits too long to convert the value to float, you might end up with ints.
The Uno and the Mega can be tricky with floats, I found being very explicit when using floats works best.