Dotted half-note together with a half-note… Dorico writes a half-note tied to a quarter note, which is correct, but… should I do it with two different voices?
create the messa di voce. Shrink its length [Shift+Option(Alt)+left arrow] to the minimum (using the eight notes grid), then shrink it further using the 32nd grid value. When it stops shrinking, augment the length by one 32nd (right arrow). Then you can move the messa di voce to the left by one 32th, pressing Command+Option (or Control+Alt) and left arrow:
But a faster/better method would be to create a Playing technique that imitates a short messa di voce, so it will have a fixed length: