Are you sure they sent you a m/f pair? I'm no expert, but by looking at color and finnage they both look male to me. Generally, male kribs have longer, pointier fins than the females and even when paled out due to stress or shipping, the female should have a noticeably brighter pink/purple belly. Males also have longer bodies and faces, where females look shorter and pudgier. I have also noticed that my male keeps his fins relaxed where my female frequently has hers more erect. Here is a picture of my pair, who are currently raising their second batch of fry (free swimming 2 days ago). The male is on the left, and the female is on the right. It is a bit blurry, but notice the female's rounded dorsal fin edge compared to the male's pointy one. Also notice how much brighter the female's belly is than the male's and how she is not quite as long and svelte as he is. If you look closely, you can see their first batch of fry in this pic as well.
If you are sure one is a female, look for the belly showing behavior. My female shows this behavior to the male constantly (except when the male is caring for their young fry). She will swim in front of him and curve her belly towards him, flashing the bright pink/purple. My male ignored her for about a week before they started digging a spawning cave together.
Post more pictures if you can and I will try to help out more.
P.S. Mine eat pretty much anything but love live and frozen foods best. Just make sure whatever you give them sinks to the bottom. Mine will NOT come to the surface to eat. You can use straw or something to stir the water so the flakes drop faster.
Tips for spawning: Do frequent water changes with slightly cooler water than what is in the tank. Mine almost always spawn the same night or the next day when I do that. Conditioning them with high protein food will help the female develop eggs faster.
Here is a pic with them and their latest batch. Fry are hard to see because they blend in well with the sand (which is a good thing!), but if you look closely you can see some tiny cream colored little guys with big eyes and spots.
Your's are very beautiful btw. Mine are still a little stressed from the shipping, so they aren't very colorful, but the females belly is still bright pink!! Hope this helps!
