First, Google Translate is horrible with Latin, and doubt is wise.
In matters of any importance I strongly suggest consulting a person who knows Latin, and this site is a good option for it.
Periculosum is just the adjective I would use for "dangerous", but the form is wrong.
The best choice is periculosi; I will discuss options below.
I would read simul more as "at the same time" than "together".
The most fitting adverb for "together" I can think of is una, but I would prefer to use a participle.
My suggestion is coniuncti, which could be translated as "connected", "married" or "together".
(Spelling variant: conjuncti.)
My complete translation suggestion is this with a rough translation:
NN et NN — periculosi coniuncti
NN and NN — dangerous when combined/married/together
I put both words periculosi and coniuncti in plural masculine nominative.
Masculine is the correct choice if at least one of the two people is male.
If both are female, then the words should be in feminine forms instead: periculosae and coniunctae.
One option to consider is to use all capital letters.
It often works well with Latin inscriptions, but the choice is of course yours.
If you do so, it is typical to replace every J with I and U with V (better leave the names as they are, though), so my alternative suggestion is:
NN ET NN — PERICVLOSI CONIVNCTI