I looked at a similar situation in Seattle last week. One vent, two toilets, with one coming in horizontally on a wye after the first. One bowl works fine, but the other toilet loses water when the other toilet is flushed. Like hj, I think that you will need to make sure that each bowl is vented. With the new toilets that flush quicker, how they are plumbed is changing. The design after the water leaves the bowl is becoming "more" important, not "less".
Flowing one toilet past the connection to a second one DOES need a vent, regardless of the required size, preferably at the point where the two meet, or else individual vents on each toilet line.