This is the most logical way bigger boats could be added. It would be very useful but also doesn't make normal boats entirely pointless. I suggest the longboat moves a tad bit slower as it has more mass.
You do not indicate explicitly: Can the 2nd player slot be used to transport mobs? (as with current boats)
More mass would argue for slower (as stated)
Doubling the oars while increasing the size by 50% would argue for faster, as would the greater keel:beam ratio (look at historical ships, long and thin >> faster), and the addition of a sail.
[Short term oars are faster than sail (which is one reason why galleys persisted so long) and more manueverable, but (in RL) sails can move a ship 24/7, rowing can't. Thus sailing ships had a higher average speed.]
It would make the coding somewhat more complex but having this slower than a rowboat (standard boat) with one oarsman and faster with two might also be considered.
Completely irrelevant to function, but the ability to add a mob head on the prow would be cool
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WARNING: I have an extemely "grindy" playstyle; YMMV — if this doesn't seem fun to you, mine what you can from it & bin the rest.