no problem at all, this is standard practice to use different tilts in 1800 and in 900.
The strategy is very often to tilt the 900 more, and tilt the 1800 less.
For example : 900 = 10° tilt and 1800 = 6° tilt.
1/ 900 are smaller
2/ 1800 are same size as these “small” 900.
The point is to reduce the 900 coverage in the city, in order to minimize 900 interference, since 900 coverage is usually very good, but the quality is not so great.
And it also allows to get a similar coverage between 1800 & 900, to be able to efficiently share traffic between both bands.
On the other hand, you can also use the same tilt for both antennas : this will allow a smaller 1800 overlap with its 1800 neighbours. Therefore, you can pack the 1800 with more TRX’s, because the frequency planning is cleaner. The 1800-cells are considered as a smaller inner zone of the 900-cells.
The strategy really depends on your network topology.
– intersite distance is small (< 500m) => tilt 900 = 8°, tilt 1800 = 6° (assumption : the vertical HPBW of the antennas is 8°)
– intersite distance is large (> 1.5km) = use same tilt for 900 (5°)and for 1800 (5° or 4.5°).
The idea is that when tilt becomes small, a difference of 0.5° makes a huge difference.