Actually, there is almost no written proof that Yehoshua bin Yosef existed, except as a literary device invented by Paul to spread reform Judaism. Most notably, the Romans, who kept meticulous records, had absolutely none about any trial, crucifixion, or other acts supposedly undertaken by Pontius Pilate. It's like he was never there at all.
Now Muhammad? We know he existed independent of his scripture. Same thing for Joseph Smith