Attenuation is usually specified for 1310 and 1550nm wavelengths.
1310nm has a higher attenuation compared to 1550nm, depending on the fiber type. Bidi uses 1270/1330nm, so even if you have sufficient power budget to establish link, there is less remaining budget to compensate for small fiber issues.
On a 60km link this would make a difference of 8-10dB (estimated) between 1310 and 1550nm. I even doubt if bidi is possible, with an average attenuation of 0.4dB/km at 1310, your link loss will be 24dB, which IS your total power budget. No addtional patch losses are included, so your link is likely to run on or out of its specifcations, unless your fibers have better specifications. But for long distance links, I've never seen usage of bidi optics.
You can't amplify signal if you need to cover some more fiber length
You only have 1 link available, no redundancy in optics or additional bandwith
You have to stock 2 pieces since both sides use specific optics (1270/1330 for side A, 1330/1270 for side B)
I highly recommend buying DWDM optics. They are not that much more expensive, but gives you much more value on the long term: You can add addtional wavelenghts to get 20Gbit (and no loss of link if a optic dies when using 2x10Gbit combined, only some loss of capacity).
Also, you can easily regenerate wavelenghts if needed, or use OADM if you need connectivty somewhere within the fiber path.
Finally, you only have to stock 1 optic as spare in case of issues instead of a set of 2. (Off course when using multiple lengths, you need to have stock for all used wavelengths)
Even if you're not using DWDM yet, you still can use them and add a mux/demux when needed at a later time.
With bidi you'll have the link (depending on the fiber type), likely running at its bottom specs, and thats all. No other options for future needs, you will have to replace them. And for 60km, there is also very little of no room for any fiber degradation or the link will run out of the specs and cause packet loss.