Entretanto vs. enquanto isso
Both entretanto and enquanto isso can be translated into English as "meanwhile" or "in the meantime", referring to an action happening concurrently with another. The main difference lies in dialectal usage and ambiguity. The phrase enquanto isso is universally understood across all Portuguese dialects as an unambiguous way to express simultaneous actions. The word entretanto readily translates to "meanwhile" in European Portuguese, but it acts primarily as the conjunction "however" in Brazilian Portuguese, meaning its temporal use requires context to avoid confusion.
Entretanto
B2When used in a temporal sense, entretanto means "meanwhile" or "in the meantime", showing that something occurs between two events or at the same time as something else. It is very common in European Portuguese to express simultaneous actions. However, it requires good context because its primary meaning in Brazilian Portuguese is "however" or "nevertheless".
Vou tomar um banho e entretanto tu podes ir adiantando o jantar.
(I am going to take a shower and in the meantime you can start making dinner.)
O médico só pode atender daqui a uma hora, mas entretanto pode ler uma revista.
(The doctor can only see you in an hour, but meanwhile you can read a magazine.)
Nós fomos comprar os bilhetes e ela entretanto guardou os nossos lugares.
(We went to buy the tickets and meanwhile she saved our seats.)
O software está a ser instalado no computador e entretanto o técnico preenche o relatório.
(The software is being installed on the computer and in the meantime the technician fills out the report.)
A Maria foi ao banco; entretanto, o pacote foi entregue pelo correio.
(Maria went to the bank; meanwhile, the package was delivered by the mail.)
Enquanto isso
B1The phrase enquanto isso translates literally to "while this" and is universally used to mean "meanwhile" or "in the meantime" in both Brazilian and European Portuguese. It is the safest and most direct choice to express that two actions are happening simultaneously, as it never diverges to mean "however".
O professor explicava a matéria e, enquanto isso, os alunos tomavam notas.
(The teacher explained the subject and, meanwhile, the students took notes.)
Eles foram ao supermercado comprar bebidas, enquanto isso nós preparamos a carne.
(They went to the supermarket to buy drinks, in the meantime we will prepare the meat.)
O pão está no forno a assar e enquanto isso vou limpar a cozinha.
(The bread is in the oven baking and meanwhile I am going to clean the kitchen.)
A tempestade destruía a cidade; enquanto isso, o governo tentava enviar ajuda.
(The storm was destroying the city; meanwhile, the government tried to send help.)
Tu vais comprar os ovos que faltam e, enquanto isso, eu vou batendo a massa do bolo.
(You will buy the missing eggs and, in the meantime, I will be beating the cake batter.)
Summary
To summarize, enquanto isso is the clearest and most universal way to say "meanwhile" or "in the meantime" in Portuguese, because it solely points to concurrent actions. The word entretanto is a great choice to mean "meanwhile" in European Portuguese, but you must be careful as it is overwhelmingly used to mean "however" in Brazilian Portuguese.