paralel updates

[passer passer]

hi.
i need your help.
why this two sql who star together are updates same record.
table engine is myisam. client work in autocommit mode.

update table set status=80925,ch_status=now() where
ins_time<now() and
now()-ch_status>300 and
status!=1
order by ch_status limit 1


update table set status=500,ch_status=now() where
ins_time<now() and
now()-ch_status>300 and
status!=1
order by ch_status limit 1

i think it is imposibl before. because updating ch_status must prevent it.



thank in advance for answers.