Increasing a number when another number gets smaller than 0 This should be trivial but for some reason I cannot think of a formula which enlarges a number in proportion to another number decreasing in the negative direction

Example:

if value 1 = $-0.1$, value 2 should be set to $0.9$
if value 1 = $-0.2$, value 2 should be set to $0.8$
if value 1 = $-0.4$, value 2 should be set to $0.6$

and so on...

Thanks for any help
How about $$\mathsf{(\text{value 2})}=1+\mathsf{(\text{value 1})}\quad ?$$
It fits all the examples you gave.
In your description you mention that one should be increasing while the other is decreasing. In your examples, both value1 and value2 are getting smaller.