Mrs and Mr His Full Name
Solution 1:
Even if your father were John Smith and your mother were Sarah Jones, the convention you speak of lists the husband’s title first and uses his name only; you’d say Mr and Mrs John Smith for the married couple together, and neither Sarah nor Jones would be mentioned in this usage. Mrs John Smith would still refer to Sarah Jones. Think of Mrs as reading “wife of” here.
This practice in which the wife’s name becomes completely invisible is no longer at all so commonly heard today as it was only a few short generations ago, but it does still occasionally surface, particularly with older people.
Sometimes young people have never have heard of it, or may even find it troubling. I would not use this style unless I already knew it to be the custom in your particular speech community.
Solution 2:
In general, it is OK to call them Mr. and Mrs. Piyush Singh, however, there are some (mostly depending on their age) who would find this discriminatory. I would suggest that you stick with introducing them as; Piyush and Seema Singh. or go fully formal and spell out their full names, Mr. Piyush Singh and Mrs. Seema Singh.