One or none mask?
#1
Is there a way to get a mask that finds both: "firstsecond" and "first second"?

And more generally: firstsecond, first-second, first second, first.second etc..
#2
I think you're looking for a rule not a mask. With a rule it's possible, see rules/T0XlC-insert_space_and_special_0_F.rule as an example.
#3
For posterity, here is an example of a some relevant rules:

:
i5,
i5-
i5.
i5 < Note SPC!

echo "firstsecond" > word
./hashcat64.bin -r rule.txt --stdout word