South Africa's Wayne Parnell and Hashim Amla are to respectively embark on spells playing for Kent and Essex.
Pace bowler Parnell is returning to Canterbury for a second time this season, starting next month.
The 19-year-old, who starred for the Proteas in their run to the semi-finals of the recent World Twenty20, impressed for the county earlier in the season.
He is returning for a five-week spell from July 10 to August 4 and will therefore be eligible for next month's Twenty20 Cup quarter-final against Durham, as well as three County Championship matches and a Nat West Pro40 game at Glamorgan.
Kent chief executive Paul Millman said: "This is great news for the club. We are delighted to be able to bring Wayne back because he made such a big impact in his initial spell.
"The team has done really well to win a tough group and qualify for the quarter-finals of the Twenty20 Cup. Wayne's return will only help our prospects for next month's quarter-final against Durham.
"It would've been nice to keep him here for a slightly longer period but the South African Cricket Board are keen for him to fulfil international commitments later in the summer."
Meanwhile, batsman Amla has signed for Essex as cover for Pakistan spinner Danish Kaneria, who has been called up by his country for the upcoming Test series against Sri Lanka.
He has agreed an initial deal until the end of next month, subject to being granted a visa and clearance from Cricket South Africa.
However, the 26-year-old could extend his stay at Chelmsford if Kaneria makes Pakistan's one-day squad.
Amla said: "I am thrilled with the prospect of joining up with Essex at this important stage of the season.
"Promotion in the county championship is a major focus for the team and I hope that I can help achieve this goal. I am really looking forward to the challenge."