You'd want to clip the less prolific of the two. If they are of equal size you can choose one or wait until one shows stronger growth and then clip the other.

it means you have a polyembryonic seed probably from a south east asian type mango.

magnificent, I have heard the exact opposite is true

Here is a link that might be useful: polyembryonic mango seedling thread

Why not separate them both and have two trees.

I've just sprouted 4 polyembrionic mango seeds and will keep the most vigorous ones alive....that makes more sense to me ..... I've been told thats how they create the Manila seedlings....

mangoD

Last year I grew several hundred polyembryonic seeds. I am interested in working out which ones are the true to type and which ones are zygotic (or off type). I understand that in the nursery industry the practice seem to be to select the most vigorous shoot and assume that it is true to type and discard the rest. I decided to keep them all, so when the seedlings were around 15cm (6") tall, I separated the seedlings and repotted them. There were generally between 4 and 8 shoots per seed. With most there were generally one or two dominant shoots. Quite a few had a noticably weak shoot which in a lot of cases later died. As the others grew on I have not really identified many as definitely zygotic. There are a few which definitely are as they exhibit different growth characteristics to their siblings such as different leaf shape. For most of the seeds it appears that it is just different vigour and I think there are quite a few variables that I have not monitored well enough to say that it is genetic or just the conditions they have grown in.
At first I thought that a very vigorous shoot may mean that it is off-type. What I think now is that it appears that the first shoot to germinate may somehow rob the large food store in the cotyledons (the fleshy part of the seed) to the detriment of the other shoots and therefore show itself to be more vigorous than the others. I have read that different varieties will give different consistency in the number of seeds which are true to type.
It seems to me that there is no simple answer to identifying the good shoots. The standard nursery practice of keeping the most vigorous shoot seems to be sound as even if it is genetically identical to the other shoots, it probably has a good connection to the food store in the cotyledon and therefore get a better start to life than one which may have lost its connection to the cotyledon.