[James.]

i figure most two pair hands are fairly robust hands HU. agaisnt a PFR we are able to narrow down his "action" holdings to a pretty tight distribution. if we 3bet the turn and bet the river and he calls we have 4 big bets go in on the big streets. this is acceptable and likely profitable in terms of action given in relation to his likely range.

but there's a problem. us 3betting the turn doesn't guarantee that we necessarily get called down every time. also, when we do get called down it's going to be from the stronger portion of his distribution and thus a lesser probality of occurrence decreasing it's frequency.

but even more importantly, if he will checkraise the turn with a one pair hand, and then bet the river and payoff a raise we come out even further ahead. our edge when the money goes in on the river will be greater than when it goes in on the turn. villain could have significant number of outs on the turn, while on the river all of the cards have been dealt and the value is essentially static.
this is additionally influenced by the fact that it's conceivable that we get paid off by a wider range of worse hands taking this line because it's a little different(so may get a wtf? i call reaction) and puts in the extra action when the board is the safest.