Hi. I bought a bonsai and unfortunately it died. Before I realised is dead I repotted it and 2 saplings grew. Now I don’t really know what to do with them. They grow quite fast and one has two leaves going brown like the original tree had. What do I have to do?Any advice please? Pics attached. Thx

It looks like your soil may not be ideal. In these pictures the soil looks quite dry and very high in organic matter. However, there are also signs of algae on the surface (the dark green patches) which suggests that the soil was too wet for too long. It is likely that you were watering too frequently. Soil mixes with a lot of organic matter hold more moisture.
I would suggest repotting this one with a soil mix that has much less organic. You can find info on this in the How To section of this website.

I was watering it every 3 days.. and that is soil special for bonsai bought from a garden centre. Bonsai potting mix as written on the bag. What should I do with those two saplings?

They are not saplings. The probably aren't seedlings either, but suckers.

I'd let them grow.

And learn how to water properly, not to a schedule

Echoing Abraham Lincoln, writing Bonsai Soil on a bag does not make the stuff inside bonsai soil.

Different plants have different requirements, and different climates have different requirements. Most everyone here mixes their own, and varies by plant, etc. My baobab has almost no organic, while the azaleas have a lot and the elms have a medium amount, because the temperature here was 104F/40C today, and the azaleas get thirsty while the baobab just wants a rumor of water somewhere in the neighborhood.

Learn about your particular tree and fit the soil to it. It might help to find a club nearby and see what others in your climate do.