API binay link help

[info at vallemediatime.com]

Hi Ruslan,
I need to create (and check) a link STUDENTS <---> PARENTS
Every student can have only two linked record on PARENTS table: one for father and one for mother.
The link is 1:M type but if I want limited M to 2 value like max value.... how can I do it?
Thanks 
Paolo
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