Simultaneous communication with deaf children
In short, practice bilingualism, not simultaneous communication.
Simultaneous communication or its term variants (SimCom and sign supported speech) is one of some "communication methods", in which one speaks both spoken language and manually coded English at the same time.
Top reasons for say no to SimCom
Speaking two languages in two modes simultaneously may sound perfectly feasible to naive hearing people. But, in truth, it's not feasible. It's unrealistic.
The original idea of this method, after realizing the failure of oralism-only for deaf education, is to teach deaf children English through a manual system, which is again a failure.
Signed English is not a language
Because, these two language ASL and English are entirely different with their own grammatical rules and structure as well as their own structure of the smallest units of language.
ASL has its own grammatical rule and structure that one cannot speak two languages at the same time, regardless of separate modalities. It's an analogue to speaking English and typing Arabic at the same time. Result? Broken Arabic with English-like grammar.
Not only it corrupts grammar in an oppressed language, it also loses intonation, contextual meanings (words in different contextual sentences), prosody, and all other linguistic features.
SimCom is a symbol of oppression
Simultaneous communication is not only unacceptable, but it's also a symbol of disrespect and history-long oppression. Using sim-com exhibits an user's ignorance and audist behavior. It may offend Deaf people (except for a few culturally acceptable situations).
When speaking both languages at the same time, one strongly tends to, if not always, speak full English and broken ASL. What this tells is that ASL tends to be devalued and that English tends to be chosen or a priority over ASL. It explicitly exhibits an audist attitude.
SimCom can be harmful to language acquisition
A number of studies, including neuroscience and linguistics, reveal that ASL or any signed language is a genuine, human language that language regardless of hands or lips is central to the brain, not speech. Below are two quick examples of some facts.
Speaking ASL activates the same linguistic regions of the brain (Broca for expression and Wernicke for recepion) as speaking English. What this means is that language resides in the brain, not in mouth or hands.
Language acquisition in ASL is on the same milestones as that in English. For example, linguistic pointing (pronouns in ASL) emerges at about the same time as pronouns in English in children. What's more, it comes with pronoun reversal errors that appear in ASL-speaking toddlers despite its iconicity at around the same age as found in English-speaking toddlers.
SimCom with deaf children during the critical period of language development hinders natural language acquisition. Because, 1) deaf children are not fully accessible to spoken language via speech, despite cochlear implants or any other devices; 2) signed English is not a language. In the end, language acquisition is weakened.
Practice bilingualism, not simcom
Approaches to speaking two languages -- English and ASL -- at home with a child is no different from those speaking two spoken languages at home.
To teach a bilingual baby a word for an object in two languages, sign an ASL word and then speak an English word separately. Balance with firsts. E.g. first ASL, then English. Next time, first English then ASL. In some cases, a monolingual parent speak one language and the other parent speaks another language. It depends from one family to another. But, don't speak both languages at the same time with a few occasional exceptions (usually with one or two words maximum).
For example, an ASL-speaking parent might sign "no" at the same time vocalizing "no" for more emphasis (e.g. to prevent a child from doing something dangerous). It's normal. It's the same with an English-speaking parent using a manual gesture to accompany a vocal word.
To articulate full sentences, use one language at a time.
Brenda Schick. "The Development of Sign Language and Manually Coded English Systems." Deaf Studies, Language and Education(2003) by Mark Marschark and Patricia Spencer (Eds). Oxford University Press.
Related signs: simcom: a cultural perspective.