Technical reproducibility in RNA-seq is considered to be excellent (provided that the same kit/lab/...) is used. So yes, technical replicates can be combined. I think the best stage to do this is as fastq or bam, although I can't think of problems of just adding the read counts.
My 2 cents: no, it's very bad practice to do that.
technical replicates are different RNASeq runs performed on the same sample, they have to be averaged. You are taking a snapshot of the transcriptional profile of your sample, and you're sequencing it twice to avoid biases and to reduce the chance to fall for sequencing errors.
The same can be said for biological replicates, but in that case you are sequencing more than one sample, to avoid batch effects and sample preparation errors to bias your downstream analysis.