Replicate opacity animation to every title in my sequence
I tried using Cross Dissolve but I can't have it more than once in the same sequence (and I don't know if it works both for fading in and out).


I'm not sure what you mean there. You can add transitions to the beginning and end of every clip (a title is considered a clip in the context of premiere), or respectively to every cut between adjacent clips. In fact, if you only want the titles to fade in and fade out, this would be the preferred method. Far easier than doing it with keyframes. Two quick tips regarding this approach:


You can quickly add the default transition1 to multiple titles/clips by selecting them in the timeline (select multiple clips by holding CTRL and left-clicking on them) and pressing CTRL + D.
You can set the default length of transitions in frames or seconds in the program preferences under Preferences → General → Video Transition Default Duration. This will also affect transitions added using the shortcut described above, but changing this setting will not change transitions that are already applied to your clips.





  How can I repeat this without having to create keyframes and set their opacity manually for every title?


Once you have created the keyframes for the first title clip, right-click on it and select Copy from the context-menu. Then, select all the title clips that you want to add the same keyframe animation to in the timeline and right-click on one of them. Select Paste Attributes… from the context menu. This will open a context window where you can select which effects you want to paste (In your case, select Opacity. You might also have to select Scale Attribute Times. Note that this will work with any effect you have added to your clip, not just the default ones). Click OK and it will apply the selected effect settings to all the selected clips.





1 The default transition is highlighted with a blue border in the effects panel. You can change it by right-clicking on the desired transition in the effects panel and selecting Set Selected as Default Transition.