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Why No Lots For The Afternoon Tomid?
The gemara says in Yoma  that although lots were taken to determine which Kohen would perform the various avodos, no lots were taken for the afternoon Tomid. Whichever Kohanim performed the morning Tomid would also merit performing the afternoon Tomid.
Sadly, there is little or no explanation as to WHY?
Two Tomids - Two Mizvos Or One?
Maybe it is rooted in a machlokes rishonim. The Rambam holds that the two Tomids are one mitzva while the Ramban holds that they are two separate mitzvos. According to the Rambam it makes sense that the same Kohanim who were involved in the morning Tomid should also be involved in the afternoon Tomid so that they don't do half a mitzva.
But then we have to understand why according to the Ramban we don't have separate lots for the afternoon Tomid which is a distinct mitzva. We also have to understand why according to the Rambam separate lots are taken for the afternoon Ktores when he holds that it is a separate mitzva. It is true that it is a segulah for wealth but that should not justify forcing the Kohen to do a half a mitzva...
Rambam: One Complete Mitzva Done Twice
It could be that according to the Rambam the morning Tomid and Ktores are really distinct, complete mitzvos, but when it comes to counting them in the minyan hamitzvos they are only enumerated as one mitzva. In fact we learn in the Mishna in Menachos  that the morning Tomid is not מעכב the afternoon Tomid. So it is one mitzva that is done twice a day.
Does Time Define The Mitzva
The machlokes between the Rambam and Ramban is whether the time is part and parcel of the nature of the mitzva. The Ramban holds that time is part of the essential nature of the mitzva and therefore the mitzvos [Ktores and Tomid] performed at different times are counted separately while the Rambam holds that the time doesn't determine the nature of the mitzva and therefore they can be counted as one mitzva [that is performed twice a day].
How Can Shma Be Said All Day And All Night And Still Be זמן גרמא?
The Kesef Mishna [we could all use some of that...] famously holds that the mitzva of morning Krias Shma is [מן התורה] all day and the night Krias Shma all night. The Magen Avraham says that if that is correct then it is not a מצות עשה שהזמן גרמא because it is done for all 24 hours. The PROBLEMO is that the gemara is explicit [Brachos 20] that it IS a מצות עשה שהזמן גרמא.
Shaagas Aryeh - Not Zman Grama Because They Are Done At Different Times
The Shaagas Aryeh  comes to the rescue and roars that it IS a מצות עשה שהזמן גרמא because the mitzva must be done twice at separate times. The would seem to correspond nicely with the opinion of the Ramban who holds that [as by the Ktores and Tomid] the time is part and parcel of the nature of the mitzva and thus it is really two mitzvos performed at different times and not זמן גרמא. But according to the Rambam, it would not seem to work out so well, because he holds that time doesn't define the mitzva and we are doing the exact same mitzva [which is not defined by it's time frame] twice a day [albeit at different times]. According to his opinion it would seem to be non-zman-grama because it is essentially a 24/7 mitzva [done twice every 24].
Tashlumin For Shma
There are opinions that maintain that there is תשלומין for Krias Shma just as there is for Tfilla [see S.A. 58/7]. This would seem to fit with the Rambam that Krias Shma is one mitzva done twice. So if he missed the morning let him make it up at night [as he would a missed Tfilla]. But according to the Ramban that they re two completely different mitzvos [because the mitzva is defined by the time] then [as they say in Latin] מהיכי תיתי that you could have תשלומין?? Tfilla can have תשלומין because it is בקשת רחמים for which there is a decree that there is a makeup time which is NOT THE CASE regarding Krias Shma. You can't fix one mitzva by doing a different one....
Why No Lots For Afternoon Tomid?
Back to our original question.... It could be that just as the gemara says that we have 4 separate lots in order to fulfill the pasuk אשר יחדיו נמתיק סוד בבית אלקים נהלך ברגש - that it is honor for the house of Hashem that there is "action" numerous times a day [see Yoma 24b], there is also a law of יחדיו נמתיק סוד that all of the Kohainm should be present for the taking of the lots and if we have separate lots for the morning and afternoon Tomids then maybe some won't show up for the morning lots [planning to be there for the lots for the afternoon Tomid] and we will lack the יחדיו.
Rashi [14b] doesn't seem to have the beginning of the pasuk [אשר יחדיו] in his text but maybe our text is telling us a huge chiddush, namely that there is a special din of יחדיו, that all of the Kohanim should be present for the lots.
[עפ"י שארית יוסף להרב הגאון ר' שלמה וואהרמן ח"ד סי' א]