Siem had a 4-under-par 71 in his second round and Dodt turned in a 70, leaving them both at 6-under 136 after 36 holes. They have a one-stroke lead over Thomas Bjorn, who shot a 68 Friday.
Five golfers are tied for fourth at 4-under. That group includes Peter Lawrie, who was the only golfer among those in the Top 20 after the second round to shoot bogey-free golf Friday.
First-round leader Chris Wood had three double bogeys Friday en route to a 5-over 76 and dropped into a tie for 21st at 1-under 141.
Siem opening round Thursday was very consistent as he opened with 16 consecutive pars and had birdies on two of the last three holes. But Friday he had seven birdies and three bogeys.
Dodt had four birdies and three bogeys, including two of his first three holes of the second round.
Swim Week Miami Beach 2014 [PHOTOS]