Q:

Sum of 2 different twig variables

I have 2 twig variable:
One is in $
The second is a double 
{{ var1 + var2 }} is giving an error:
eg.
var1 = $5.00

var2 = 10.00

A:

You're adding a float var2 on to a string var1. You can remove the dollar sign from var1 like such.
{% set var1 = var1|replace({ '$': '' }) %}
{{ var1 + var2 }}

You may also need to target &dollar; as well as $
{% set var1 = var1|replace({'$': '', '&dollar;': ''}) %}

Edit: As suggested in comments of your original question by @iainn, you should attempt to make this uniform rather than a mixture of non-formatted and formatted values.

