Read serial 8 bytes from Arduino buffer
The standard way of doing this is to use delimiters. Serial is a streaming protocol, you don't necessarily get data in fixed size chunks. Something suitable would be like:

&lt;1.23,4.56&gt;


Notice the delimiters "&lt;", "," and ">".

The receiving end ignores incoming data until it gets "&lt;". Then it reads until it gets a comma. Between those two are the first number. Then it reads again until it gets a ">". Between those are the second number. Now you validate that what you got are in fact numbers.

This allows you to easily expand in the future to larger numbers, and more decimal places, eg.

&lt;42.213,674.50326&gt;