debug log mentions this :
2014-09-13 10:50:35 Darksend is idle
2014-09-13 10:50:35 Submiting Darksend for 600.000000 DRK
2014-09-13 10:50:35 DoAutomaticDenominating -- attempt 1 connection to masternode 54.68.102.109:19999
2014-09-13 10:50:35 trying connection 54.68.102.109:19999 lastseen=15169.0days
2014-09-13 10:50:36 connected 54.68.102.109:19999
2014-09-13 10:50:36 send version message: version 70038, blocks=30770, us=217.122.139.212:19999, them=54.68.102.109:19999, peer=563
2014-09-13 10:50:36 DoAutomaticDenominating --- connected, sending dsa for 60000000002
2014-09-13 10:50:36 receive version message: /Satoshi:0.10.13.8/: version 70038, blocks=30770, us=217.122.139.212:58747, them=54.68.102.109:19999, peer=563
2014-09-13 10:50:36 send initial getblocks peer=563
2014-09-13 10:50:36 getblocks -1 to 0 limit 500 peer=563
2014-09-13 10:50:36 dssu - state: 2 entriesCount: 0 accepted: 0 error: no matching denominations found for mixing
2014-09-13 10:50:36 CDarkSendPool::UpdateState() == 3 | 2
2014-09-13 10:50:36 CDarkSendPool::UpdateState() == 2 | 7
2014-09-13 10:50:36 Darksend request incomplete: no matching denominations found for mixing. Wll retry...
2014-09-13 10:50:46 CDarkSendPool::Check() -- RESETTING MESSAGE
2014-09-13 10:50:47 Darksend is idle
2014-09-13 10:51:10 ProcessMessage(dsq, 120 bytes) FAILED
2014-09-13 10:51:10 ProcessMessage(dsq, 120 bytes) FAILED
2014-09-13 10:51:10 ProcessMessage(dsq, 120 bytes) FAILED
2014-09-13 10:51:10 ProcessMessage(dsq, 120 bytes) FAILED
2014-09-13 10:51:10 ProcessMessage(dsq, 120 bytes) FAILED
2014-09-13 10:51:10 ProcessMessage(dsq, 120 bytes) FAILED
2014-09-13 10:51:10 ProcessMessage(dsq, 120 bytes) FAILED
2014-09-13 10:51:10 ProcessMessage(dsq, 120 bytes) FAILED
2014-09-13 10:51:10 ProcessMessage(dsq, 120 bytes) FAILED
2014-09-13 10:51:10 ProcessMessage(dsq, 120 bytes) FAILED
2014-09-13 10:51:10 ProcessMessage(dsq, 120 bytes) FAILED
2014-09-13 10:51:12 dssu - state: 2 entriesCount: 0 accepted: -1 error:
2014-09-13 10:51:12 dssu - message doesn't match current darksend session 0 823432
2014-09-13 10:51:36 Submiting Darksend for 600.000000 DRK
2014-09-13 10:51:36 DoAutomaticDenominating -- attempt 1 connection to masternode 188.226.223.5:19999
2014-09-13 10:51:36 trying connection 188.226.223.5:19999 lastseen=15169.0days
2014-09-13 10:51:36 connected 188.226.223.5:19999
2014-09-13 10:51:36 send version message: version 70038, blocks=30770, us=217.122.139.212:19999, them=188.226.223.5:19999, peer=564
2014-09-13 10:51:36 DoAutomaticDenominating --- connected, sending dsa for 60000000002
2014-09-13 10:51:36 receive version message: /Satoshi:0.10.13.8/: version 70038, blocks=30770, us=217.122.139.212:58752, them=188.226.223.5:19999, peer=564
2014-09-13 10:51:36 send initial getblocks peer=564
2014-09-13 10:51:36 getblocks -1 to 0 limit 500 peer=564
2014-09-13 10:51:36 dssu - state: 2 entriesCount: 0 accepted: 0 error: no matching denominations found for mixing
2014-09-13 10:51:36 CDarkSendPool::UpdateState() == 3 | 2
2014-09-13 10:51:36 CDarkSendPool::UpdateState() == 2 | 7
2014-09-13 10:51:37 Darksend request incomplete: no matching denominations found for mixing. Wll retry...
2014-09-13 10:51:38 dssu - state: 2 entriesCount: 0 accepted: -1 error:
2014-09-13 10:51:38 dssu - message doesn't match current darksend session 0 843542
2014-09-13 10:51:47 CDarkSendPool::Check() -- RESETTING MESSAGE
2014-09-13 10:51:47 Darksend is idle
conclusion : it combines my 100 and my 500 amounts to 600 so they can both reach round 8
but then cant find a pair for 600 .. which is logical as amount 600 is very rare to pair with.....