I have not seen cable of any sort in direct contact with concrete. It has always been placed in conduit, in the gravel bed. At the least it has been UF in the gravel bed, but never low voltage in direct bury.
In my opinion, you would be wise to place 2 conduits under the level of the concrete. This with keep the concrete from cracking over the pipe, it acts as a control joint, but on the underside if into the concrete. The 2 conduit need to be separated at least by 6" more if possible. This will keep the low voltage from picking up on the interference created by the line voltage.
Your length of run, should be good, in the States there is a maximum run of 200 ft between pull boxes (Maybe it is 100'), one of the electrical guys can firm this I hope. Never the less, you are ok on this.
If you are using direct bury cable, after you get out from the edge of the concrete, I would get it down 18" if possible, or keep it in the conduit to protect it from casual digging.
TL;DR -- go right ahead
This looks to be a sound plan -- it's definitely cheaper than running everything in conduit, and probably somewhat easier too even with having to stuff two NM cables down a 3/4" EMT length.
Fill isn't an issue
Even though the Code conduit fill criteria don't apply to protective sleeves, you're fine on fill (albeit barely). Encore Wire quotes .160" by .450" for their 12/2 W/G NM product -- multiplying and converting to mm^2 gives us a conservative area estimate of 46.5 mm^2 per cable. Doubling this to account for the doubled run gives us 93mm^2, which is within the 106mm^2 (31%
fill) available for two wires or cables in the 3/4" EMT.
Nor are derates
Normally, since you have more than 3 current-carrying conductors in the sleeve, you'd have to derate as per 310.15(B)(3)(a). However, this derate starts with the 90°C column in Table 310.15(B)(16) as per 334.80. Since we are derating by a factor of 80% for the 4-6 conductor case, we get 24A derated, which is above the 20A 60°C ampacity that's actually used for the cable as per 334.80, so we're good on that front too.
And the EMT is suitable
NEC 334.15(B) states that EMT is one of the allowable means for protecting NM from damage in an exposed application. PVC can also be used, but it must be Schedule 80 PVC -- the cheaper/wimpier Schedule 40 stuff just won't do. There shouldn't be any concerns with impedance, grounding, or the likes either -- protective sleeves are not required to be grounded or bonded as per 250.86 Exception 2, and it's grounded anyway via being connected to a grounded metal box.
Best Answer
National Electric Code calls for protection of the cable or raceway (conduit).
National Electric Code (NEC)
Building codes dictate how large the holes and notches can be.
International Residential Code (IRC)
Which means for bearing walls with 2x4 studs, the maximum hole diameter is 1 3/8" and maximum notch depth is 7/8". For a non-bearing wall with 2x4 studs, the maximum hole diameter is 2 1/8" and maximum notch depth is 1 3/8". For bearing walls with 2x6 studs, the maximum hole diameter is 2 3/16" maximum notch depth is 1 3/8". For non-bearing walls with 2x6 studs, the maximum hole diameter is 3 5/16" maximum notch depth is 2 3/16".
Check local codes for amendments before proceeding. It might also be a good idea to consult with an engineer, to insure structural integrity is maintained.