Khaled Abdulaziz Al Eid, individual bronze medallist at the 2000 Sydney Olympics, and Abdullah Waleed Sharbatly were banned in May after their horses tested positive for controlled medication substances at events in February.
The CAS said in a statement that the sole arbitrator had partially upheld their appeals at a hearing in London and, considering the infraction was of minor importance, reduced the bans to two months which have already been served.
An International Equestrian Federation spokeswoman said both men would be able to compete in London providing they had achieved minimum eligibility requirements by a June 17 deadline.
Sharbatly was individual silver medallist at the world equestrian games in 2010.
Their horses Vanoeve and Lobster 43 tested positive for the non-steroidal anti-inflammatory drugs phenylbutazone and oxyphenbutazone which are listed as controlled medication substances, said the FEI last month.
Such substances are prohibited in competition but permitted for treatment use outside competition.