Don't worry, the photos I have are not the best (and they are not mine ), but I have the text
Both altars.
His brother:
And in this spanish forum, you can find a lot of photos of the recently opened museum (including the source of theo ones I have linked here)
Text:
C(aii) CAESARIS. AVGVSTI.F(ilii)
PONTIF(icis).CO(n)S(ulis).DESIG(nati)
PRINCIPIS.IVVENTVTIS
[L(ucius).IVNIVS.L(ucii).F(ilius).T(iti) N(epos). PAETVS.
[SAC(rum)]
[D(onum)].D(edit)
Dedicated to the children of Agrippa and Julia, the "young princes", to which the theatre was dedicated.
I think that another altar with the name of his brother is lost. Something like "L Caesaris. Avgvsti...".
In fact, the Frons pulpiti is resolved with the 3 known altars to Jupiter and... I don't remember (Minerva?). The 3 principal gods, in spanish "triada capitolina", the 2 altars you have seen now, and another 2 similar altars. I haven't linked photos to the 3 altars to Jupiter and company.
The other altar:
L(ucius) IUNIUS L(ucii) F(ilius) T(iti) N(epos)
PAETUS
FORTUNAE SAC(rum) D(onum) D(edit)