Rules for combining activities in the lab require all activities to eb the same (e.g. Enchant Lesser Device) and that they are of the same Tech and Form. But they may be different devices.
And a tiny note: Enchanting from a labtext (orm inventing spell for that matter) requires your Lab Total for exceed the level, not just match it.
Yes you could enchant to devices from the text. But you need a Lab Total of 38+, since you need 19+ for each project.
No, this is not a possibility by RAW. Well, technically you could try and micro-manage and use rules for Distractions to get a penalty to Lab Totals for mising time (but these are huge, and there is a limit to how many day can be missed for it to count as a season in lab) and combine it with reduced time to read books - which is linear IIRC - so missing 1 month (1/3) of the time reduced Story Total by 1/3. My recommendation: Don't do it!
I figure this should be errata'd, especially since the following sentence states "he may not use the Laboratory Text until his Lab Total increases to be at least equal to the level", which wouldn't make sense if the lab text were still useless if your lab total exactly equals the level of the effect.
I noticed that too, but the sentence you refer to also doesn't state that the lab total needs to add up to the total of all levels plus 1 for each of the effects - so in that case dedicating part of the lab total exactly equal to the level of the desired effect is enough as well.
If someone had a 37 lab total, and wanted to make two things that were 18 with the benefit of a lab text, I'm down with that.
There are other examples of this inconsistency. Starting out (just after gauntlet or advancing some time post gauntlet) a character can learn a spell equal to the TeFo (page 32), and this is confirmed when learning a spell from another, the limit is equal to the lab total in the TeFo (page 95).
So the Lab Total being equal to seems to appear more often than it needing to be greater than.