Mitochondrial genomes differ greatly in size, coding potential and even whether they are circular or linear. Mammalian mitochondrial DNA is small (11–28 kbp) and intronless. However the mitochondria of certain other organisms range up to 1000 kbp in size.
Certain sponges (demosponges) with large mitochondrial genomes contain type I introns and type II ...
I think that you are talking about trans-splicing. This does indeed happen. It is fairly common in protist organisms, though in humans it is quite rare. For more information about how this process works, including mechanisms in vertebrate organisms, see this paper.
Here is one model from that last paper for how it works:
There are two different ways to interpret this question:
Does the spliceosome splice all introns of an mRNA simultaneously or sequentially?
Can a spliced mRNA be re-spliced?
How does a spliceosome proceed along the mRNA?
According to this review:
splicing generally proceeds in a 5′ to 3′ direction, although neighboring introns are not always excised in ...