You could do it like this.
x <- vector("character", length(ano)*length(bimestre))
k <- 0
for (i in 1:length(ano)){
for(j in 1:length(bimestre)){
k <- k + 1
x[k] <- paste(base_url_rreo,
"an_exercicio=", ano[i], "&",
"nr_periodo=", bimestre[j], "&",
"co_tipo_demonstrativo=", tipo_relatorio, "&",
"no_anexo=", num_anexo, "&",
"id_ente=", ente, sep = "")
}
}